1. Field of the Art
The present invention relates to a vacuum valve and, more particularly, to a vacuum valve which may be suitably used in a vacuum sewer system in which soiled water, e.g., domestic waste water, is transported by vacuum.
2. Prior Art
Since conventional gravity flow type sewer systems suffer from the problem that the maintenance of conduit tends to be costly, vacuum sewer systems have recently attracted attention. In general, a vacuum sewer system is composed of three parts, that is, a soiled water basin or house inlet equipped with a vacuum valve, a vacuum sewer pipe, and a vacuum pump station.
Domestic soiled water is collected by gravity flow in the soil water basin equipped with a vacuum valve. As the liquid level in the soiled water basin rises to a predetermined level, a vacuum valve control mechanism detects it and opens the valve. Thus, the soiled water collected in the soiled water basin is sucked into the vacuum sewer pipe. The vacuum valve continues to remain open for a predetermined period of time even after the soiled water has been sucked in, and air is also sucked in during this period. The soiled water in the pipe is pushed by the expanding air in the pipe and carried to a vacuum pump station in the form of a mixed-phase flow. When a certain amount of soiled water has been collected in a collecting tank, the water is sent to a sewage treatment plant of a public sewage system by a soiled water booster pump.
The conventional vacuum valve that is used in the above-described vacuum sewer system, however, suffers from the problem that since the gap between valve body and the inner wall surface of the valve casing is small, when the valve is open or closed, bulk foreign matter, e.g., pebbles, is likely to get caught in the gap, thus interfering with the movement of the valve body. Since the above-described gap is constantly uniform along the stroke of the valve body for the purpose of guiding the valve body, there is always a likelihood of foreign matter getting caught in the gap when the valve body operates. If the valve body remains in a half-open position because foreign matter has been caught in the gap, the vacuum in the entire system is destroyed, causing a breakdown in the vacuum sewer system.
To solve the above-described problems, the present applicant has proposed a vacuum valve such as that shown in FIG. 5 as Japanese Patent Application No. 4-336571 (1992) (Japanese Patent Public Disclosure No. 5-240373). The vacuum valve V has a casing 31, a valve body 35 disposed in the casing 31 in such a manner as to be movable obliquely, and a valve rod 36 for supporting the valve body 35.
The casing 31 has a bottom wall 31a, and a side wall 31b which obliquely extends from the bottom wall 31a. The casing 31 has an inlet opening 32 and an outlet opening 33. The axes of the inlet and outlet openings 32 and 33 lie along a horizontal straight line. In addition, a valve seat 34 is formed on the bottom of the casing 31.
Thus, the casing 31 has the casing side wall 31b expanding outwardly in a direction approximately perpendicular to the axis x of the valve from a position near the valve seat 34 over 360 degrees around the axis x. By this arrangement, bulk foreign matter, e.g., pebbles, flowing in from the inlet opening 32 are prevented from getting caught between the valve body 35 and the casing inner wall. The degree of expansion of the casing side wall 31b is so set that when the valve body 35 is released from the position where it rests on the valve seat 34, the distance between the outer peripheral portion of the valve body 35 and the casing inner wall is 0.8D or more (D is the bore of the inlet opening 32).
The vacuum valve having the above-described arrangement is capable of extremely effectively preventing bulk foreign matter, e.g., pebbles, with a diameter not larger than 0.8D from getting caught between the valve body 35 and the causing inner wall. However, the conventional vacuum valve suffers from the problem that, as shown in FIG. 5, elongate foreign matter F, e.g., disposable wooden chopsticks, is likely to get caught between the casing inner walls. To ascertain the cause of the problem, the present applicant repeatedly conducted experiments while elongate foreign matter was allowed to flow in the vacuum valve. As a result, it was found that in the improved casing proposed in Japanese Patent Application No. 4-336571 (1992), the casing has a front wall portion A where the casing inner wall is at an obtuse angle to the direction of a streamline S connecting the inlet and outlet openings, as shown in FIG. 5. Therefore, if elongate foreign matter F, e.g., disposable wooden chopsticks, comes into contact with this portion, the head of the elongate foreign matter is pushed up along the wall surface by a soiled water flow. As a result, such foreign matter gets caught between the casing walls.